In 3GPP Long Term Evolution (LTE) mobile communication systems, a base station and a terminal apparatus establish an initial connection and perform communication using the following procedure.
The base station first transmits a search signal (discovery signal) such as PSS a primary synchronization signal (PSS)/secondary synchronization signal (SSS) and a broadcast channel signal (hereinafter referred to as “broadcast signal”) such as a physical broadcast channel (PBCH). In this case, the base station transmits a search signal so that the search signal can be received in a whole cell area covered by the base station. In other words, a point at which it is not possible to receive the search signal transmitted from the base station is outside of the cell area covered by the base station because the terminal apparatus cannot detect the base station.
Here, the search signal is a reference signal sequence known to the base station and the terminal apparatus and is used for the terminal apparatus to detect the presence of the base station, detect a cell number, detect radio frame timing or measure receiving quality. The search signal is also referred to as a “synchronization signal” or “beacon signal” or the like. The broadcast signal includes information on a radio frame number and a system bandwidth of the base station.
The terminal apparatus detects the presence of the base station by receiving the search signal and measures receiving quality. When detecting a plurality of base stations, the terminal apparatus determines a base station having the highest receiving quality among the detected base stations to be the connection destination. Next, the terminal apparatus transmits a random access channel (RACH) signal (hereinafter referred to as “RACH signal”) which is a connection request to the base station determined to be the connection destination. LTE provides 64 preamble sequences as signal sequences for random access channels. The terminal apparatus randomly selects one preamble sequence from among the 64 preamble sequences and transmits the preamble sequence to the base station. Upon receiving the RACH signal, the base station transmits a random access response (RAR) (hereinafter referred to as “RAR signal”) which is an acknowledgment response to the terminal apparatus which is the sender of the RACH signal. The RAR signal includes radio resource allocation information for the terminal apparatus and identification number (Temporary cell radio network temporary identity (C-RNTI)) assigned to the terminal apparatus. The terminal apparatus performs communication with subsequent base stations using the radio resource allocation information and the identification number indicated by the RAR signal (Non-Patent Literature (hereinafter referred to as “NPL” 1).
In recent years, small base stations that carry out communication using a millimeter wave band are being introduced to secure a bandwidth of 1 GHz or greater in response to a drastic increase in traffic demand. Since such a small base station uses the millimeter wave band, the reachable distance of a radio wave cannot be extended and propagation loss increases significantly.
As a scheme that contributes to overcoming such propagation loss, enhancement of communication speed and expansion of a cell area, there is a scheme in which the base station performs directivity control (beam forming) using a plurality of antenna elements (antenna array). According to the scheme that performs directivity control, a radio wave transmitted by the base station is directed toward a direction in which the terminal apparatus is located, and the radio wave can thereby reach a farther point than non-directional transmission and the cell area to be covered can be expanded. Moreover, since the terminal apparatus can improve a signal to interference-plus-noise power ratio (SINK), it is possible to use a modulation scheme and a coding rate with high frequency utilization efficiency, and thus to carry out communication at a high transmission rate (NPL 2).
However, since the base station has no information for determining the direction in which the terminal apparatus is located at a stage before establishing an initial connection, it is not possible to direct the directivity toward only the direction in which the terminal apparatus is located to transmit a search signal.
Thus, the LTE mobile communication system proposes a method that divides a range intended to be used by the base station as a cell area into small portions according to the width of the directivity and transmits a search signal while successively switching the directivity so that directivity is directed toward each of the divided ranges. According to the method, it is possible to expand a coverage area through directivity control while covering the whole range to be set as the cell area by successive operation.